There is a difference between a version of map data before updating map elements such as a road and a version of map data after updating the map elements. A known technology delivers the difference as map difference data to a vehicular navigation unit. The vehicular navigation unit receives the delivered map difference data and updates map data based on the map difference data.
For example, patent literature 1 discloses a technology that delivers map difference data to a vehicular navigation unit. According to the technology, the map difference data is generated in units of map elements, rather than in units of meshes into which a map data layer is divided. To update map elements that are dependent on each other, the technology disclosed in patent literature 1 groups the map elements by dating back to update of the map elements with map data of different versions and generates map difference data to avoid a conflict on a road network. When the dependence exists between map elements, updating one map element requires updating the other map element to prevent the former from conflicting with the latter.